The place C++ will have at Unity:

C++, C# and Unity

by Lucas Meijer

From the article:

C++ is not great at this task. I want my loop to be vectorized, but a million things can happen that might make the compiler not vectorize it. It might be vectorized today, but not tomorrow if a new seemingly innocent change happens. Just convincing all my C/C++ compilers to vectorize my code at all is hard.